[llvm-libtool-darwin] Add constant CPU_SUBTYPE_ARM64_V8

Add support for constant MachO::CPU_SUBTYPE_ARM64_V8. This constant is
needed so as to match `llvm-libtool-darwin`'s behavior to that of
cctools' libtool when `-arch_only` flag is passed in on command line.

Reviewed by jhenderson, alexshap, smeenai

Differential Revision: https://reviews.llvm.org/D85041